An electrolytic capacitor generally has a structure including an anode of a valve metal such as aluminum or tantalum, a dielectric of an oxide film formed on the surface of the anode, a cathode, and an electrolyte disposed between the cathode and the dielectric. Here, an oxide film is formed by an anodic oxidation method in which an oxide film is formed on the surface of a metal using the metal as an anode in an acid electrolyte or neutral electrolyte. In addition, in the present application, the term “electrolyte” includes both solid and liquid forms. For example, in an example using aluminum, a thick porous oxide film is formed in an acid electrolyte such as sulfuric acid, oxalic acid, or phosphoric acid and a thin, dense, barrier-type film is formed in a neutral electrolyte such as borate, phosphate, or adipate. A porous aluminum oxide film is used for anticorrosion or antifriction of a metal or for decoration by coloration. A barrier-type film is broadly used as a dielectric of an electrolytic capacitor.
The electrolyte in such an electrolytic capacitor has two important roles. One is a function as an actual cathode which achieves the task of taking out capacitance from a dielectric on an anode and is therefore required to have high electric conductivity, i.e., high electron conductivity. The other is a function for protecting and repairing an extremely thin oxide film by a chemical action for newly forming an oxide at a defected portion of an oxide film of aluminum or tantalum based on the ion conductivity of the electrolyte. The above-mentioned anodic oxidation method is used for the purposes of forming a dielectric oxide film in an electrolytic capacitor and repairing a defected portion of an oxide film. Therefore, it is necessary that the electrolyte of an electrolytic capacitor has anodizability.
In general, an organic or inorganic acid or a salt thereof is added to an organic solvent, such as ethylene glycol or γ-butyrolactone, and the mixture is used as the electrolyte of an electrolytic capacitor. Examples of the added organic or inorganic acid or a salt thereof include phosphoric acid, formic acid, acetic acid, ammonium adipate, ammonium succinate, tertiary amines, and quaternary ammonium salts. These composite electrolyte systems are used for obtaining an electrolyte excellent in ion conductivity.
However, though the conductivity of these liquid electrolytes is improved by adding an additive described above, it is insufficient for realizing a low impedance capacitor. In addition, in these liquid electrolytes, a dry up phenomenon is caused by evaporation of the used solvent. If the dry up occurs, both anodizability and conductivity are lost, resulting in a low heat resistance and a short life-time, which is a problem.
In order to solve these problems, a molten salt has been investigated to use as an electrolyte for a capacitor. For example, it has been investigated to constitute an electrolyte for a capacitor by, without using a solvent, melting or melting and then solidifying an electrolytic salt composed of a nitrogenous heterocyclic cation having a conjugated double bond or composed of a nitrogenous heterocyclic ring having a conjugated double bond.
Furthermore, it has been investigated to constitute a capacitor by interposing an electrolyte alone or with a separator between an anode foil and a cathode. The electrolyte for the electrolytic capacitor is in a molten state prepared by mixing carboxylate and carboxylic acid not using a solvent. However, since such an electrolyte is a solid at ambient temperature, the anodizability is significantly decreased and the conductivity property is poor. Therefore, the practical application has not been achieved yet.
On the other hand, recently, a capacitor using a solid electrolyte not containing solvents (called solid electrolytic capacitor) has been developed. Specifically, one or more electrically conductive polymers such as polypyrroles, polyanilines, polythiophenes, polyquinones, derivatives thereof, polymers prepared by polymerizing an aromatic compound containing an amino group, and polymers prepared by polymerizing an aromatic compound containing a hydroxyl group are used as an electrolyte. These conductive polymers have a significantly higher electric conductivity (electron conductivity) compared with those of liquid electrolytes using the above-mentioned known solvents. Therefore, in capacitors using these polymers as electrolytes, the internal impedance can be decreased. In particular, capacitors used in high frequency circuits show excellent characteristics. Therefore, these conductive polymer capacitors have been occupying an important position in the electrolytic capacitor market.
However, these conductive polymers do not essentially have ion conductivity and therefore are far inferior in the anodizing function for repairing an oxide film of an electrolytic capacitor to known capacitors using liquid electrolytes. It is generally thought that, in a conductive polymer capacitor, the dielectric film is prevented from breakage by insulating the conductive polymer present on the dielectric surface at the breakage portion by a dedoping reaction caused by Joule heat generated when the dielectric film is broken. This mechanism is different from that for repairing an oxide film of a capacitor using a known liquid electrolyte in the fundamental principle.
Consequently, a conductive polymer capacitor has a disadvantage that a high withstand voltage cannot be obtained. Specifically, in a conductive polymer capacitor using aluminum as an anode, the withstand voltage of the capacitor is about 16 V only by the forming at 70 V, for example, and in a conductive polymer capacitor using tantalum, the withstand voltage is about 12 V only by the forming at 34 V, for example. Here, the term “forming at 70 V” means that a direct-current voltage applied to a valve metal which becomes an anode when a dielectric oxide film is formed on the surface of the valve metal, i.e., a forming voltage (or referred to as applied voltage, the same applies hereinafter), is 70 V. Logically, it is possible to increase the withstand voltage by using a higher forming voltage. However, in such a case, the capacitor capacitance is decreased with an increase of the forming voltage, and the withstand voltage is not increased in proportion to the increase in the forming voltage. Thus, it is not a good method.
As an attempt to improve withstand voltage characteristics of such a conductive polymer capacitor, an electrolytic capacitor using an electrolyte composed of a conductive polymer and an organic acid onium salt is disclosed (for example, see Japanese Unexamined Patent Application Publication No. 2003-22938 (hereinafter referred to as Patent Document 1)). However, this organic acid onium salt is fundamentally thought to be a salt in a solid state. Therefore, in order to improve withstand voltage characteristics, the ratio of a conductive polymer (P) and an organic acid onium salt (O) is preferably (P): (O)=1:0.1 to 5, more preferably (P): (O)=1:0.2 to 2. However, in the range of this ratio, the withstand voltage characteristics are certainly improved, but the conductivity is worsened. This undesirably deteriorates the impedance characteristic of the capacitor.
Recently, remarkable molten salts which are liquids at room temperature (for example, 10 to 30° C.) have been developed independently of the technologies relating to the above-mentioned electrolytic capacitors. These are called an ionic liquid and are composed of a combination of a proper cationic component (quaternary salt cation such as imidazolium or pyridinium) and a proper anionic component (Br−, AlCl4−, BF4−, or PF6−). Many ionic liquids contain a halogen. These ionic liquids are characteristically nonvolatile, nonflammable, chemically stable, and highly ion conductive and are regarded as remarkable reusable green solvents which are used in chemical reactions such as various synthesizes and catalyst reactions. However, it has not been reported that the ionic liquid is investigated from the viewpoint of anodic oxidation, namely, from the viewpoint of forming an oxide film on the surface of a valve metal or repairing an oxide film.
Further, ionic liquids are generally hydrophilic except for ones containing some anions such as PF6− or (CF3SO2)2N−, and moisture may isolate a hazardous gas from an ionic liquid. For example, in 1992 Wilkes and Zaworotko disclosed [EMIm][BF4−], but it is hydrophilic and consequently has a limitation in its application field. Since these hydrophilic ionic liquids have a property to absorb moisture, an electrolytic capacitor using such a hydrophilic liquid as the electrolyte is reduced in water resistance and moisture resistance and is decreased in electric property, which is also a problem.
In addition, claim 25 of PCT Japanese Translation Patent Publication No. 2004-527902 (hereinafter referred to as Patent Document 2) discloses “A long-lived electrochemical device comprising in combination:    (a) a conjugated polymer working electrode;    (b) a counter electrode;    (c) an ionic liquid having an anion and a cation in contact with both said working electrode and said counter electrode; and    (d) a power supply for applying a voltage between said working electrode and said counter electrode, whereby a response is induced in said electrochemical device.”In Patent Document 2, the conjugated polymer (polyanion) is limited to ones electrochemically deposited on the electrodes and is characteristically an anion of an ionic liquid.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-22938    Patent Document 2: PCT Japanese Translation Patent Publication No. 2004-527902